Dielectric heating has been well-known in the heating art for many years. Cooking, bonding, drying and similar processes are accomplished by subjecting a dielectric material to a high frequency alternating electric field that produces heat in the material resulting from molecular friction. The field penetrates the material such that, even in the interior the heating process substantially begins at the instant the electric field is applied. This is contrasted to conventional conduction heating where thermal energy is gradually conveyed from the surface of the material toward the center and the interior remains "cold" for some period of time. The field penetration thus described has the advantage of heating much more rapidly than by conduction but it also has the disadvantage associated with it of difficulty in controlling the depth of penetration. Particularly in the application of cooking, it is important to have the surface of the food maintained at a high enough temperature for a period long enough to brown or sear the surface of meat.
A number of techniques have been used in the microwave heating art to achieve a browned food surface while cooking. One method includes the incorporation of auxiliary electric or gas broiling elements. Another approach is to coat the food with an additive that exhibits a dielectric characteristic producing increased food surface temperatures. Another concept is presented in U.S. Pat. Nos. 3,857,009 and 3,946,187, both issued to the inventors herein and assigned to the assignee of the present invention. They disclose structures which mount in a microwave field and upon which food is placed. The microwave energy induces currents in the structures having intense alternating electric fringing fields in close proximity of the food with said fields rapidly decaying a short distance from the food surface. However, in the dielectric heating art, a cooking means is still required for controlling the depth of penetration for individual preferences and a variety of foods with different shapes and sizes.